Respirators that filter air for breathing are frequently worn when individuals work in areas where air may be contaminated with toxic or noxious substances. Filtering respirators may operate under negative pressure, in which inhalation by the wearer draws air through the filter, or they may operate using positive pressure, in which a fan or other device supplies air to a wearer. A powered air purifying respirator (PAPR) typically includes a motor blower unit, a filter, and a power source (e.g., a battery pack). In some systems, these components reside on a belt around the waist of a user, with a tube conveying the purified air to a facemask, hood, helmet, etc. Belt or back-mounted equipment may, however, be subject to disengagement from the wearer during use and/or may also reduce the ability of the wearer to work in tight spaces.
Some PAPRS, such as the AIRSTREAM.TM. and AIRHAT.TM. (3M Company, St. Paul, Minn.), house these components inside a helmet to avoid the need to mount components on the user's back or belt. It is advantageous to locate the filter in helmet-mounted PAPRs within the crown space between the wearer's head and the helmet's outer shell. Locating the filter within the crown space can help to reduce the profile or size of the helmet as compared to helmet-based systems in which the filter is located outside of the crown space. Helmets with smaller profiles are desirable because they allow the wearer to work in smaller, tighter spaces than helmets with larger profiles.
Powered air purifying respirator performance is measured by parameters such as airflow, pressure drop across the filter during operation, efficiency in removing contaminants, and particulate loading capacity. Airflow and pressure drop are related because, for a given blower and power source, a filter with a smaller pressure drop will deliver higher airflow. Conversely, a filter with a larger pressure drop will deliver lower airflow using the same blower and power source. Airflow and pressure drop are important because, to provide the same amount of filtered air, a respirator system with a higher pressure drop filter requires more energy than a respirator system with a lower pressure drop filter.
Pressure drop for a given airflow rate across a filter can be decreased by increasing the openness or looseness of the filter material. A filter in which the openness or looseness of the filter material is increased, however, typically exhibits reduced efficiency in removal of contaminants, which is another one of the parameters by which powered air purifying respirator system performance is measured. Pressure drop for a given airflow rate can also be reduced without decreasing the efficiency of contaminant removal by increasing the size or surface area of the filter. Increasing the filter size, however, typically also includes increases in the size and/or bulk of the system. Such increases in size and/or bulk of the filter can increase the profile or size of helmet-mounted PAPRS, thereby potentially limiting the wearer's mobility in confined areas.
Helmet-mounted and other powered air purifying respirator systems used in connection with dust/mist filters have included filter bag holders that support the dust/mist filter bag inlet and/or the perimeter of the dust/mist filter bag. One such respirator system is disclosed in, e.g., U.S. Pat. No. 4,280,491 (berg et al.). The dust/mist filter bag holders typically provide only limited or no support through the center of the dust/mist filter bag because the airflow alone was sufficient to prevent kinking of the dust/mist filter bag materials.
Another filter support used in some helmet-mounted respirators is illustrated in FIG. 1. The filter bag holder 110 is designed to support a flat dust/mist filter bag in an arcuate form to fit within the crown of a helmet. The holder 110 is constructed of two members 112 and 114, with the smaller member 114 being held in compression to provide an opening 116 between the two members at one end thereof. Both members 112 and 114 include a plurality of openings 118 and 120, respectively, that are aligned along the length of the holder 110.
The filter bag holder 110 is manufactured of relatively flexible thermoformed polystyrene and is designed to allow a filter bag to billow outward from the filter support during operation. In other words, the filter bag holder 110 is designed primarily to maintain the filter bag in an arcuate shape.
New regulations promulgated by the National Institute of Occupational Safety and Health (NIOSH) for powered air purifying respirators, require the respirators to use High Efficiency Particulate Air (HEPA) class filters that remove 99.7% of a test aerosol. See 42 C.F.R. .sctn.84 (1995). To provide that level of contaminant removal efficiency, however, the filter materials used are typically significantly stiffer and/or heavier than the filter materials used in lower efficiency dust/mist filters.
Filters that provide HEPA class efficiency in helmet-mounted powered air purifying respirators can be provided in relatively rigid pleated media. When the pleated filters are located in a helmet's crown space, the pleated filter media are typically arcuately shaped as seen in, e.g., U.S. Pat. No. 4,462,399 (Braun). Shaped, pleated filters typically include the desired filter material along with the support structure required to maintain the shape of the pleats. The additional components increase the filters' cost and also typically increase the filters' bulk, resulting in higher shipping and storage costs. The helmet used with pleated filters may also be larger, thereby increasing the profile and limiting the wearer's maneuverability in confined spaces.
As compared to shaped pleated filters, filter bags are typically less expensive to manufacture, and are also less expensive to ship and store due to their smaller size. When used in connection with helmet-mounted respirators in which the filter bag must be arcuately-shaped to fit within a helmet crown, however, the stiffer material in flat HEPA class filter bags may tend to kink. Filter bag kinking may be particularly severe if the HEPA class filter bag is supported only about its perimeter as with some known filter bag holders.
Kinking creates flow obstructions that can reduce filter bag performance by increasing the pressure drop across the filter which can, in turn, reduce the airflow rates of the respirator system using the kinked filter bag. Filter bag kinking can also reduce the filter's effective area which, in turn, can also reduce the filter particulate loading capacity. As discussed above, particulate loading capacity is another one of the parameters by which filter performance is judged. Kinked filter bag areas are not evenly loaded with particulate matter during use, thereby reducing filter bag particulate loading capacity.
The filter bag holder 110 of FIG. 1 is another example of known filter bag holder designed for use with dust/mist filter bags and is typically incapable of preventing kinking of HEPA class filter bags. In addition, the holder 110 is formed of members that have relatively large surface areas. Those large surface area members can effectively block airflow to large areas of the filter, thereby increasing pressure drop and reducing particulate loading capacity.